The invention relates to a laser with a beam path multiply folded by means of two cavity resonator end mirrors and with at least one reflector retroreflectively folding the laser beam and whose axis is in particular laterally displaced with respect to the axis of a further reflector.
In the case of the conventional laser, a plurality of reflectors retroreflectively folding the laser beam is constructed as a roof mirror, and two such adjacent members are axially parallely displaced relative to one another in the folding plane by approximately half their total width or approximately by half a reflecting mirror width, so that the laser beam path reciprocates and can be progressively folded. Thus, these roof mirrors are components of a so-called multipass resonator, whose folding mirrors or reflectors are used for reducing the longitudinal extension of the laser. Folding generally leads to problems in the beam guidance and quality. The beam guidance is impaired in that the beam is not completely precisely reflected in the desired direction, because the mirror adjustment is not accurate or can be undesirably influenced by the laser design. The beam quality is impaired, for example, by diffraction effects during reflection in the corner regions of roof mirrors. Generally speaking, the disadvantages increase with the number of folding actions.
A general improvement with regards to the aforementioned disadvantages is obtained through the use of retroreflective mirrors including the aforementioned roof mirrors. Such retroreflective mirrors have the property of reflecting parallel to itself an incident beam independently of the angle of incidence, so that its adjustment sensitivity is comparatively small. Such retroreflective reactors act in one plane, such as e.g. the aforementioned roof mirrors, or three-dimensionally, i.e. if three reflecting surfaces are arranged at right angles to one another. Although the adjustment sensitivity of, for example, 10 millirad is significantly reduced compared with other, non-retroreflective reflectors of, pay, 100 microrad, the beam quality still needs improvement.
An object of the present invention is to so improve a laser such that, while maintaining a comparatively low adjustment sensitivity, it has an improved beam quality.
This object is achieved by the present invention in that the beam path is folded at least twice with the retroreflective reflector.
It is an important aspect of the present invention that the influence of optical inhomogeneities on the beam quality can at least partly be removed by the special folding geometry of the retroreflective reflectors. In addition, the adjustment sensitivity is further reduced with such reflectors. This is mainly achieved in that the retroreflective reflector is utilized several times for folding the beam path. Thus, there are no optical imprecisions between individual, independent reflectors. The adjustment errors of several individual retroreflective reflectors are avoided and the stability is increased. Optical inhomogeneities are, in particular, reduced compared with conventional multipass resonators, in that spatial superimposing of individual beam path portions can be avoided.
Advantageously, the offset of the reflectors is the same or greater than the radius of the laser beam and smaller than the radial extension of the reflector mirror. As a result of this dimensioning of the offset it is ensured that there is no reciprocal overlap of beam path portions. By avoiding overlaps of parallel beam path portions of the laser beam, the best possible beam quality is obtained. The laser beam remains in the folding area of the two reflectors. In addition, the laser beam does not have to radiate into the corner area of a reflector. Such corners of two or three-dimensional retro-reflective reflectors lead to pronounced diffraction effects because the corners of the reflectors cannot be made precisely at right angles. There are instead always rounding effects and unevennesses bringing about further resonator losses leading to a significant divergence angle of the radiation reflected by them and, at the same time, significantly impairing the beam quality. Moreover, the corner of the retroreflective reflector is generally exposed to a risk of damage, so that a beam guidance avoiding this corner generally avoids the risk of any deterioration of the laser illumination.
The retroreflective reflector acts in a two-dimensional manner, and the offset occurs in the folding plane. As a result of this construction of the laser or its resonator arrangement, it is possible to achieve a favorable utilization of the active laser material in one plane. An optimum utilization of the active material in space or in three directions diverging from one another is achieved by the retroreflective reflector acting in a three-dimensional manner and by the offset being determined by a translatory displacement of the reflector adapted to the available active laser material cross-section. In both aforementioned cases, the above-described advantages are accompanied by an increased flexibility during the adaptation of the reflectors to the constructional circumstances of the laser through a corresponding choice of offset. In addition, the utilization of the active material is more effective, i.e. a performance improvement occurs as compared with conventional multipass resonators.
Advantageously there are two at least optically facing retroreflective reflectors, and each reflector folds the beam path at least twice. Such a construction is particularly advantageous for a simple construction of retroreflective reflectors acting in two or three-dimensional manner because there is then at least approximately equal folding and consequently reflectors of the same size are used.
In order to completely use the space between two retroreflective reflectors and to hold the active laser material with maximum compactness, the laser is constructed in such a way that a resonator end mirror is arranged equiaxially with a retroreflective reflector or axially parallel thereto in the sense of a spiral-shaped expansion of the beam path. It is appropriate for the resonator end mirror to be arranged in the corner area of the reflector which, as a result of the aforementioned disadvantages, should not in any case be incorporated into the beam path, so that a further improvement to the compactness of the laser construction is obtained.
It is also very important for obtaining a compact laser construction to use a space of the active laser material extending radially in all directions. In the case of planar or two-dimensionally acting retroreflective reflectors this can be achieved by two two-dimensionally acting reflectors allowing retroreflective folding in at least two beam path planes and at least one retroreflective reflector arranged in an edge manner with respect thereto and bringing about the transfer of the beam path between the two beam path planes. In order that the retroreflective reflectors can be used in an optimum manner for beam folding, the two resonator end mirrors are arranged laterally outside the beam path plane alongside one of the retroreflective reflectors and level with one of the beam path planes.
If there are two-dimensional retroreflectively acting reflectors and a translatory offset bringing about two parallel beam path planes, the number of separate reflecting components of the resonator arrangement having several beam path planes is further reduced. It is possible to eliminate one or more mirrors for passing the beam path between two beam path planes.
Since in particular in the case of a multiple folding of the beam path, the laser radiation or illumination does not always have negligible divergences or power losses, it is advantageous to fit in the beam path at least one intermediately focussing optics device. The construction of the optics device is matched to the beam guidance arrangement. Advantageously its construction is a parabolically acting mirror deflecting all the beam path portions, or a collecting lens extending over the entire cross-section taken up by the folded beam path. For reasons of easy manufacture, the parabolically acting mirror is more particularly used for a beam path folded in one or more planes, whereas preference is given to the collecting lens if the beam path is so constructed with respect to the cross-sectional plane that the production of parabolically acting mirrors is too complicated or leads to an inadequately accurate beam guidance and an unacceptably deteriorated beam quality.
While utilizing the aforementioned features, the laser of the present invention can, in particular, be so constructed that it permits a beam division. It is equipped in such a way that a resonator end mirror is arranged as a partly transmitting window within the folded beam path and that the end mirror is followed by at least one further partly transmitting output or outcoupling mirror. The laser illumination leaving the resonator is so guided between the retroreflecting reflectors outside the resonator area that part of the laser illumination is coupled out and the reflected portion is available at another point as a second beam. The beam division achieved in this way is in particular of interest because the illumination portion leaving the resonator is again passed by the retroreflective reflectors through active laser material and can consequently be amplified both before and after reaching the or one outcoupling mirror.
An advantageous construction is obtained if both resonator end mirrors are arranged equiaxially and at least one reflecting surface of one of the reflectors forms the output mirror. In this case, all the beam path portions formed by the reflectors are available for amplifying the complete laser illumination or part thereof and the beam quality and adjustment sensitivity are not impaired by an increased number of components.
According to another aspect of the invention, the laser has a dielectric material laser block having channels adapted to the active material in the beam diameter, and the retroreflective reflectors are positioned at a distance from the block or on correspondingly shaped end faces.